Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor wafers, being utilized in a CD metrology tool, the so-called CD-SEM (critical dimension scanning electron microscope), and a defect review SEM. In a SEM, the region of a sample to be examined is two-dimensionally scanned by means of a focused primary beam of electrically charged particles, usually electrons. Irradiation of the sample with the primary electron beam releases secondary (and/or backscattered) electrons. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected electric current. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
SEM typically includes such main constructional parts as an electron beam source (formed with a small tip called “electron gun”), an electron beam column, and a detector unit. The detection unit may be located outside the path of the primary beam propagation through the column, or may be located in the path of the primary beam (the so-called “in-column” or “in-lens” detector). The electron beam column includes inter alia a beam focusing and collecting arrangement formed by a lens arrangement and a deflector. The deflection of the primary beam provides for scanning the beam within a scan area on the sample, and also for adjusting incidence of the primary beam onto the sample (an angle of incidence and/or beam shift).
One of the common goals of all imaging systems consists of increasing the image resolution. In SEM, in order to reduce the “spot” size of the electron beam up to nanometers, a highly accelerated electron beam is typically produced using accelerating voltages of several tens of kilovolts and more. Specifically, the electron optic elements are more effective (i.e. produce smaller aberrations) when the electrons are accelerated to high kinetic energy. However, in order to avoid damaging a sample (resist structure and integrated circuit) that might be caused by such a highly energized electron beam, the electron beam is decelerated just prior to impinging onto the specimen. Deceleration of the electrons can generally be accomplished by selectively creating a potential difference between the pole piece of a magnetic objective lens and the specimen. Alternatively, the same effect can be achieved by actually introducing electrodes having selective potential applied thereto.
Another known problem of the inspection systems of the kind specified is associated with locating defects (foreign particles) on patterned surfaces. The pattern is typically in the form of a plurality of spaced-apart grooves. To detect the existence of a foreign particle located inside a narrow groove, it is desirable to tilt the scanning beam with respect to the surface, which tilting should be applied to selective locations on the specimen. A tilt mechanism may be achieved by mechanically tilting the sample holder relative to the charged particle beam column, and/or by electronically tilting the primary beam propagation axis.